The invention relates generally to implantable biological tissue stimulators, and the specific embodiment relates particularly to advanced implantable cardiac pacers employing microcomputer technology, telemetry and antitachycardia systems.
The major pumping chambers in the human heart are the left and right ventricles. Simultaneously contracting these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from smaller antechambers called the left and right atria which contract about 100 milliseconds (ms) before the ventricles. This interval is known as the atrioventricular (AV) delay. The contractions are induced by a wave of spontaneous electrical excitation which begins in the right atrium, spreads to the left atrium and then enters the AV node which delays its passage to the ventricles via the so-called bundle of His. The frequency of the waves of excitation is normally regulated metabolically by the sinus node. The atrial rate is thus referred to as the sinus rate or sinus rhythm of the heart.
Electrical signals corresponding to the contractions appear in the patient's electrocardiogram. A brief low amplitude signal known as the P-wave accompanies atrial contraction, normally followed by a much larger amplitude signal, known as the QRS complex, with a predominant R-wave signifying ventricular contraction. Repolarization prior to the next contraction is marked by a broad waveform in the electrocardiogram known as the T-wave.
A typical implanted cardiac pacer operates by supplying missing stimulation pulses on a pacing lead attached to the ventricle. The electrical stimulus independently initiates contraction. The R-wave can be sensed by the same lead and used as a timing signal to synchronize or inhibit stimulation pulses in relation to spontaneous cardiac activity. The atrium also can be sensed and/or stimulated by a separate lead lying in the atrial appendage. In AV sequential pacers both atrial and ventricular leads are used for sequential stimulation of the atrial and ventricular chambers. An example is shown in U.S. patent application Ser. No. 207,003 entitled "Multi-Mode Microprocessor-Based Programmable Cardiac Pacer", filed Nov. 14, 1980 by Leckrone et al, (hereinafter referred to as the 207,003 application), assigned to the assignee of the present application.
Although usually only supplementing cardiac function, cardiac pacers can be life supporting devices. They are surgically implanted and remain inside the patient's body for many years. Malfunctions are rare but the mere possibility of requiring surgical replacement dictates a conservative approach, if not reluctance, toward exploiting new developments in electronic circuitry. In the past, the relatively straightforward functional requirements were successfully implemented even with analog hardware circuit configurations.
The state of the art in compact batteries has also been a major factor. Current drain must be minimized to avoid unnecessary surgical replacements and reprogramming of an expensive new pacer.
Reliability is the chief concern, however, followed closely by low current drain and compactness. Driven largely by the latter concerns, the industry gradually moved to adopt low power digital integrated circuits and, most recently, microprocessors. Examples of microprocessor-based pacers are contained in the 207,003 application, U.S. patent application Ser. No. 430,507, entitled "Multi-Mode Microprocessor-Based Programmable Cardiac Pacer" filed Sept. 30, 1982 by William Vollmann and U.S. patent application Ser. No. 195,665 filed Oct. 9, 1980 by Alan Lesnick, entitled "Implantable Externally Programmable Microprocessor-Controlled Tissue Stimulator", all three assigned to the assignee of the present application. Microprocessor technology presents the challenge of writing a pacing routine which monitors sense amplifier outputs indicative of spontaneous activity of the heart and safely determines and provides the type of stimulation that would be best suited to a given condition. The main problem in exploiting this technology is safety. To be sure, the boundless complexities of computer programming give rise to new opportunities for versatility. However, residual design faults all too often exist as evidence of the designer's inadequate mastery of the complexity of their system.
The interest in microprocessors for tissue stimulators intensified following the introduction of the first low power CMOS single chip microprocessors. One of the first complete implementations is documented in the 195,665 application in which a microprocessor is used in a neural stimulator for alternating lead connections and reprogramming stimulation parameters. In this configuration, a separate timer is employed to time fixed stimulation pulse intervals. Meanwhile, a slow CPU clock keeps the microprocessor running to respond to interrupts. When the timer times out, a fast CPU clock is substituted to manipulate the lead configuration. In contrast, the computer-based pacers of the 207,003 and 430,507 applications depend on software execution for interval timing. A standard scan cycle (14 ms) is established in the pacing routines. No matter what sequence of decisions and actions are taken, the pacing routine scan cycle always consumes the same number of machine cycles. Tailoring every possible software path through the pacing routine to the same number of machine cycles requires wasteful delay loops and a less than optimum clock rate.
Among the variety of cardiac symptoms encountered by cardiologists, one of the most complex is tachyarrhythmia. In atrial tachycardia, for example, the sinus rate accelerates uncontrollably to 180 to 300 beats per minute (bpm). Atrial tachycardia response modes are disclosed in the 207,003 and 430,507 applications. In the fallback mode, the pacer rate is increased to a point just below the maximum rate. Every few seconds the rate is decreased by 1 scan cycle (14 ms) until it reaches the programmed fallback rate. A programmable option is 2:1 AV block at atrial rates greater than the maximum rate which is set equal to the atrial referactory period. These systems do not attempt to break or interfere with the patient's tachycardia.
Overdrive pacing systems requiring external intervention have been used to attempt to break tachycardias. See, for example, U.S. patent application Ser. No. 243,135 entitled "Cardiac Pacer Apparatus" filed Mar. 12, 1981 by Peter Tarjan assigned to the assignee of the present application. Systems of this kind are called antitachycardia mechanisms to differentiate them from mere tachycardia response modes.
Noninvasively programmable cardiac pacers have become widely accepted over the past ten years and are now considered a necessity in most applications. RF or magnetic impulse transmission allows an external programmer to enter new pacing parameter data in the pacer's registers by coded transmissions. More recently, outbound telemetry systems have been devised to allow the implant to retransmit parameter information to the outside programmer to report the current parameters and confirm reprogramming. Expanding outbound telemetry to include patient data is another one of the subjects of the present invention.